An X-ray system typically includes an X-ray tube. The X-ray tube generates X-rays across a vacuum gap between a cathode and a rotating anode structure. In order to generate the X-rays, a filament driving circuit generates thermo-ionic current from the cathode. In releasing of the electrons, the filaments contained within the cathode are heated to incandescence by passing an electric current therein. The electrons are accelerated by the high voltage potential and impinge upon the anode, whereby they are abruptly slowed down to produce X-rays in the form of an X-ray beam.
The high voltage potential across the vacuum gap is typically on the order of 140 kV. Although the filament driving circuit is operated at this high voltage potential, the actual voltage between the terminals of filament driving circuit leads is low and is approximately on the order of tens of volts. Even though the driving circuit is often isolated and the filament wires are insulated, as a result of the voltage difference between the high voltage potential across the vacuum gap and the low operating voltage of the driving circuit, overvoltage transients occur therein.
The overvoltage transients can also be caused from floating high voltage structure, discharges caused by insulator surface contamination, and filament shorting in the cathode. Floating high voltage structure refers to bad contacts on the cathode and between the cathode and the driving circuit. The overvoltage transients occur in the form of discharges or spits. Abnormal discharges occur when the filaments are temporarily or permanently shorted to the cathode cup, which may be in the form of pin-to-pin discharges. The discharges can cause degradation to the minor insulation on the leads and the cathode cable terminals.
The spit activity causes radiated and conducted electrical noise of high intensity, which can interfere with operations of electronic circuitry in the vicinity of the tube, to the extent of the X-ray system becoming inoperative. Also, the insulation between the filaments is only capable of protecting against voltage potential discharges of approximately between 1 kV and 5 kV, thus, the filament insulation can also breakdown from the spit activity. Acceleration of insulation breakdown increases over time and can cause the X-ray system to operate inappropriately and eventually become inoperative.
The overvoltage transients can also cause high voltage degradation to the feedthrough insulators and breakdown in the minor insulation of a high voltage cable between the cathode and the driving circuit. The loss of high voltage integrity between filaments and common in the high voltage cable can result in instable or uncontrollable high voltage regulation.
The overvoltage transients are especially critical in mono-polar X-ray tubes that have a relatively higher power capacity. The overvoltage transients are more predominant in the mono-polar tube applications and therefore, X-ray tube component degradation is also more predominant.
Thus, there exists a need for an apparatus that prevents the occurrence of overvoltage transients within an X-ray tube.